Instructions
In-Game
Press the "PS" button, which is the round button located in the center of the PS3 controller. The XMB menu will appear.
Select "Reassign Controller" from the XMB menu and press the "X" button.
Select a controller assignment number from the displayed list. Press the "X" button to confirm the controller number reassignment.
Press the "O" button to close the XMB menu screen and return to your game.
On the PS3 Home Screen
Press and hold the "PS" button in the center of the PS3 controller until the XMB screen appears.
Select "Turn off controller" from the XMB menu and press the "X" button. The PS3 controller will disconnect from the PS3 and turn off. The XMB menu screen will close automatically.
Press the "PS" button to turn the controller back on. The controller will reconnect with the PS3 and take the lowest available controller assignment number.